Railroad hopper cars carrying bulk particulate matter, such as grain or plastic pellets, are typically unloaded by applying a vacuum conveying line to an outlet gate positioned at the bottom of each car compartment. The primary air flow for the vacuum conveying line is obtained from the exterior of the car. That is, the exhaust air for unloading is not drawn from the car compartment through the particulate matter. Nevertheless, the top of the car compartment must be vented to compensate for material drawn out the bottom of the compartment. Failure to vent the top of the compartment reduces the efficiency of the vacuum unloading process and even risks damage to the car's structure. Most commonly, venting for unloading hopper cars has been achieved by propping open at least one hatch cover on each hopper compartment being unloaded.
Opening hatch covers presents disadvantages. First, some means permitting safe access to the top of the railroad car must be provided. However, doing so, unduly complicates the unloading process. Secondly, open hatches tend to invite security problems. Not only is there a risk of contaminating the lading due to the open hatch, but also empty compartments are tempting disposal sites for refuse after the lading has been removed. Further, unless the hatch cover is secured by hinges and is lockable, the cover may be misplaced or stolen.